GN: update error message for invalid input_conversion.

tools/gn/input_conversion.cc

Index: tools/gn/input_conversion.cc
diff --git a/tools/gn/input_conversion.cc b/tools/gn/input_conversion.cc
index 043737a72ac8a2e22ababcd70d6574069eb18ae4..c14f4ae6f767e456ed035b72a4a4b8006fba8f6b 100644
--- a/tools/gn/input_conversion.cc
+++ b/tools/gn/input_conversion.cc
@@ -140,7 +140,7 @@ Value DoConvertInputToValue(const Settings* settings,
     return ParseValueOrScope(settings, input, PARSE_SCOPE, origin, err);
 
   *err = Err(original_input_conversion, "Not a valid input_conversion.",
-             "Have you considered a career in retail?");
+             "Run gn help input_conversion to see your options.");
   return Value();
 }